1. Field of the Invention
The present invention relates to an electric Al or Al alloy plating bath which can be used at ordinary temperature. More particularly, the present invention relates to an electric Al or Al alloy plating bath for forming an electric Al or Al alloy plated layer, which can be used as a usual surface treatment for the prevention of the occurrence of any corrosion.
2. Brief Description of the Related Art
It has been well-known that an aluminum metal material shows excellent anti-corrosive properties, but aluminum has a strong affinity for oxygen and the reduction potential thereof is inferior to that of hydrogen. For this reason, the electro-deposition of an aluminum layer from an aqueous solution containing the same is quite difficult. Therefore, the electro-plating of aluminum has long been put into practice while using an organic solvent-based plating bath or a high temperature molten salt bath. Typical examples of such organic solvent-based plating baths include those obtained by dissolving AlCl3 and LiAlH4 or LiH in ether; those obtained by dissolving these components in tetrahydrofuran; and solutions of NaF.2Al(C2H5)3 in toluene. However, these baths suffer from a problem such that the handling thereof is quite difficult, since it may involve a risk of causing an explosion when it is brought into contact with the air or water. Thus, there has been proposed a mixed molten salt bath comprising an aluminum halide and an alkylpyridinium halide as a bath free of any risk of causing an explosion (see Patent Document 1 specified below). Moreover, there has also been proposed a molten salt bath comprising an aluminum halide and an alkyl imidazolium halide, which is further blended with a zirconium halide (see Non-patent Document 1 specified below). However, the plating of aluminum from such an Al—Zr alloy plating bath results in the formation of an electro-deposited layer which is non-uniform and insufficient in the smoothness. In particular, when increasing the thickness of the plated layer and/or when increasing the current density, a problem arises such that a dendritic deposit is formed at high current density portions and the deposit thus formed is easily peeled off from the surface of a substrate. Contrary to this, when reducing the current density used, another problem arises such that the throwing power is reduced and this accordingly results in the formation of areas free of any deposit layer. Moreover, if the resulting plated film is subjected to, for instance, the salt spray test without subjecting the film to a chromate-treatment which makes use of chromium (VI)-containing compound, the film is easily dissolved in the salt solution, never ensures the expected anti-corrosive power and accordingly, it would be quite difficult to obtain a highly anti-corrosive Al or Al alloy plated film. To solve such a problem associated with the foregoing molten salt bath, the inventors of this invention has previously proposed a method for diluting such a molten salt bath by the addition of a general-purpose aromatic solvent such as benzene, toluene or xylene (see Japanese Patent Application No. 2007-030553). The addition of, for instance, benzene, toluene or xylene would permit the improvement of even the throwing power without unreasonably reducing the electric conductivity of the molten salt. When using such an aromatic solvent in an industrial scale, however, a problem arises such that one should take measures to ensure the safety of human body because of their high volatility and that it is also necessary to take measures against the risk of catching fire because of their high flash point.    Patent Document 1: JP-A-62-70592    Non-patent Document 1: Journal of The Electrochemical Society, 2004, 151(7), C447-C454 (2004).